1. Field of the Invention
The present invention relates to controlled oscillators, and more particularly, the present invention relates to a controlled oscillator that provides an output frequency controlling signal subject to a noise reduction circuit.
2. Description of the Related Art
The use of wireless communication applications for in-home, in-building networks and direct communications is increasing. These applications modulate data onto at least one carrier and transmit data as a modulated signal via a transmitter. A receiver receives the frequency modulated signal, and demodulates it to recapture the data.
In the various stages of transmitting and receiving signals, local oscillators generate signals that result in an output frequency. Sometimes the desired frequency may vary. Variable frequency oscillators are tunable oscillators that receive a fixed frequency that is output over a selectable wide range of frequencies. To oscillate, a resultant gain should be high, any feedback should be positive, and the coupling from the output to input should be effective. The oscillation frequency is controlled via the frequency tuned circuits. Oscillation should take place at a predictable and stable frequency, without the introduction of excessive noise or degradation of any signals.
The frequency for a variable frequency oscillator can be adjusted via a varactor. A varactor may include a semiconductor diode that works as a variable capacitor when it is reverse-biased. The capacitance of the varactor depends on the reverse-bias voltage. The greater the voltage, the lower the value of the capacitance. The varactor is placed in a parallel with the tuning capacitor, and is isolated for direct current by blocking capacitors. The resulting oscillator may be referred to as a voltage controlled oscillator. Frequency control using the varactor is preferable to the use of mechanically variable capacitors or inductors. Varactors tend to be less bulky and lower in cost.
A voltage controlled oscillator (VCO) circuit may operate at high frequencies, such as in the gigahertz range. Noise within circuits operating at high frequency can impede performance and degrade the control and output signals. For example, noise in the input signal may result in large variations of the resulting output signal. This action, in turn, degrades performance of any circuit, detector, carrier, transmitter device and the like using the VCO circuit. Noise within VCO circuits can come from many sources. Input noise may arrive at the VCO from a coupled power supply or other components, either in the current or voltage. In a fully integrated chip, the power supply, or the chip itself can be noisy.
Noise also arrives at the VCO circuit from the gate, or reference circuit. This noise may occur when the current source is also connected to a current mirror, or bandgap reference. For example, the current mirror is in parallel with the VCO circuit. The current mirror draws current from the current supply coupled to the VCO circuit. This cross connection produces noise that is then input into a gate coupled to the VCO circuit.
Thus, noise is a critical factor in VCO circuits, particularly those that operate at high frequencies. The noise may be amplified to affect output signals correlating to the input signals driving the VCO circuit. Further, supply and bias noise is introduced to the input signals to impede the input signals.